A piston engine was just one example. There are dozens of things they could make out of printed parts that would demonstrate the fortitude and longevity of printed material. There is also literally nothing as volatile/controversial as making guns out of them though. It just seems like needlessly drawing attention, and in turn bad press, to something that does not need to be drawn attention to.

Likewise they could easily get guns elsewhere nearly as easily. Plus there's the factor that other people, stores, etc could just as easily have a gun from 3d printing too.

It doesn't matter that you could go on the internet right now and order 10 replacement barrels and have them delivered to your home with no problems, no background checks, no special paperwork, no serial numbers, no different than if your mom was ordering 10 candle holders. When you put printing and guns into the mix it becomes an entirely different thing.

Between ordering [probably electronic], payment [also electronic] and shipping [a record of some sort from where and to where and the company can tell what was in the order, including quantity] there will be some trail left for law enforcement to piece together that won't exist with 3D printed barrels. With 3d printing, the quantity will be unknown, as will the date of manufacture, so "when" won't be able to be established.